Abstract: A 3-year field experiment was conducted from 2013 to 2015 during summer castor growing seasons at the coastland of Jiangsu Province. The aim of the experiment was to analyse the effects of organic fertilizer and planting densities on population growth and yield components of castor. The interactive test of three planting densities18 000 plant·hm^-2 (D1), 15 000 plant·hm^-2 (D2) and 12 000 plant·hm^-2 (D3) and three organic fertilizer application rates12 000 kg·hm^-2 (O1), 6 000 kg·hm^-2 (O₂) and 0 kg·hm^-2 (O₃) were conducted with a castor cultivar of'Zibi 8'as tested material. The investigated items included dry matter accumulation and distribution, leaf area index, chlorophyll content, spike characters and yield components. The results showed that the soil fertility could be improved significantly by using organic fertilizer for three years. With the application of organic fertilizer of 12 000 kg·hm^-2 under planting density of 15 000 plant·hm^-2 (D2O1), the contents of soil total nitrogen, available phosphorus, available potassium and organic matter significantly increased by 47.37%, 169.21%, 54.65% and 13.77% respectively compared with that without organic fertilizer treatment (D2O₃). Both leaf area index (LAI) and relative content of chlorophyll at the late growth stage was maintained at a high level under planting densities of 15 000 plant·hm^-2 and 12 000 plant·hm^-2 with the application of organic fertilizer. Total dry matter production at filling and ripening stages under medium planting density (15 000 plant·hm^-2) plus 12 000 kg·hm^-2 organic fertilizer (D2O1) significantly improved to almost similar level to that under high planting density of 18 000 plant·hm^-2 (D1O1). In addition, the distribution of dry matter in spike and crop growth rate (CGR) under planting density of 15 000 plant·hm^-2 both improved after anthesis with the application of organic fertilizer. The effective panicles per plant, grain number per plant and 100-grain weight increased with the increased rate of organic fertilizer application, which effectively compensated for low spike number at lower planting density. In this study, the crop yield under medium density (15 000 plant·hm^-2) with 12 000 kg·hm^-2 organic fertilizer (D2O1) was 3 943.77 kg·hm^-2, which was similar to that under high planting density (D1O1). It was concluded that suitable planting density in combination with the application of organic fertilizer effectively improved population growth and the reasonable distribution of dry matter, delayed leaf senescence, increased dry matter accumulation and grain yield at mature stage of caster.